1. Field of the Invention
This invention relates to an improvement of a vibration damping material made by laminating a viscoelastic rubber sheet on a constraint plate such as a metal plate.
2. Description of the Prior Art
Recently, soundproofing materials are used in a wide range of industrial fields from cars to electrical appliances to building materials. They play a great role for reducing noise which has been regarded as a serious social problem.
Among them, vibration dampers, in particular, have been remarked as materials for absorbing vibration which causes a noise. Such vibration damping materials are generally classified, from the structural viewpoint, into those of a two-layered type (unconstraint type) made by providing a viscoelastic layer of rubber or synthetic resin on a metal plate and those of a three-layered type (constraint type) made by sandwiching a viscoelastic layer between two metal plates.
A non-constraint type vibration damping material is intended to absorb vibration by converting the vibrating energy into kinetic energy by extensional/contractile deformation of its viscoelastic layer. A constraint type vibration damping material is intended to absorb vibration by converting the vibrating energy into kinetic energy principally by shearing deformation of its viscoelastic layer.
Many of conventional vibration damping materials are of a constraint type which commonly uses a hot-melt-adhesive layer also as a viscoelastic layer (alternatively, gives an adhesive property to the viscoelastic material) and sandwiches the layer between two metal plates. In this case, the use of a hot-melt-adhesive synthetic resin film as the adhesive layer facilitates continuous bonding of layers and mass-production at a lower cost. It, however, sometimes involves a problem in adhesive property and vibration damping property, and some solutions have been proposed. Examples are: the use of a thermosetting adhesive to improve the adhesive strength, the use of a rubber sheet provided between metal plates to improve the vibration damping property, and so on.
These damping materials, however, still involve some problems.
Many of vibration dampers using a hot-melt-adhesive as the viscoelastic layer, even if having an excellent vibration damping property, are originally low in adhesive strength with the metal plates. Moreover, because of their own nature, they are not resistant against heat. When the temperature reaches the melting point (or the flow temperature), two metal plates may be peeled off. Further, because of an extreme decrease in elasticity (Young's modulus of elasticity), the viscoelastic material may flow out when compression is applied.
In contrast, vibration dampers using a thermosetting adhesive as the viscoelastic layer may be improved in adhesive strength by hardening the viscoelastic layer, sometimes, however, at the sacrifice of the vibration damping capacity.
In particular, both hot-melt-adhesive agents and thermosetting agents have viscoelastic characteristics as the nature of resins. That is, the vibration damping capacity (loss factor) has a sharp peak near the glass-transition temperature. As a result, the temperature range effective for damping vibration is very narrow (dependency to temperature is large), which means that such vibration damping materials are difficult to use. Moreover, in case that resistance against oils, solvents, and so forth, must be taken into consideration, more restrictions are imposed, and the range of actual use of the damping materials is limited.
In contrast, when a rubber sheet is used as the viscoelastic sheet and sandwiched between metal plates, it is possible to obtain a vibration damper that is small in temperature dependency and excellent in vibration damping property by controlling the viscoelastic property of rubber by adjusting the mixture ratio. Such a vibration damper may have satisfactory resistance against heat, pressure, oils and solvents. However, because of restrictions in manufacturing vulcanized rubber, it is difficult to produce a vibration damper that permits continuous bonding which facilitates mass production, like the above-indicated vibration damper using a hot-melt-adhesive agent. Even if possible, it is very difficult for the rubber sheet itself to have sufficient adhesive force effective for practical use, and other adhesive agent must be relied upon. In this case, the degree of elasticity relative to the rubber sheet must be taken into consideration when selecting an adhesive agent to be used. Otherwise, however excellent is the vibration capacity of the rubber sheet, it may become a weak point in the entirety of the vibration damper and may be readily affected by the vibration damping property of the adhesive itself. This leads to a number of secondary problems which require further consideration.
In general, rubber sheets and metal sheets are bonded by vulcanized adhesive. This method, however, makes continuous fabrication difficult; for example, only cut-sheet-scaled fabrication using a hot press is possible.
It is not sufficient for a vibration damper to merely have only a high vibration damping capacity. In actual applications, they are often used in severe environments where various factors such as temperature, pressure, oils, and solvents are applied alone or in combination, rather than in relatively moderate environments at normal temperature, under no load and in the atmosphere. There is thus still demanded a vibration damper that can sufficiently meets such requirements.
Below is presented a discussion on resistance of vibration dampers against heat, pressure, oils and solvents. In order to improve the resistance, it is necessary to decrease the burden by decreasing the thickness of the viscoelastic rubber sheet so as to decrease the absolute deforming amount with compression force. In contrast, in order to obtain a high vibration damping property, it is necessary to increase the absolute thickness of the viscoelastic rubber sheet. Therefore, the use of the rubber sheet is not suitable for overcoming the inconsistent requirements.
More specifically, an increase in thickness of the viscoelastic rubber sheet improves the vibration damping property but decreases the thermal and pressure resistance. Of course, a decrease in thickness improves the thermal and pressure resistance but reduces the vibration damping property which is inherently required to the vibration damper.
Since the vibration dampers discussed above have an advantage in one sense and a disadvantage in other sense, one or two of factors, such as high vibration damping property (low temperature dependency), thermal and pressure resistance, resistance to oils and solvents, and adaptability to continuous fabrication, must be chosen for each intended use.